FIG. 1.

Schematic diagram of the fla/che operon that starts with dual promoter elements dependent on σ^D and σ^A holoenzymes. The σ^D-dependent promoter (P_D-3) lies 133 bp upstream of the σ^A-dependent promoter (fla/che P_A), which lies 70 bp upstream of the translational start codon for the first gene in the fla/che operon, flgB. The sigD gene is found 26 kb downstream of these promoters, between the cheD and ylxL genes. DNA sequences amplified by PCR and cloned into pGEM-Zf7(+) for riboprobe synthesis or pJM102 for insertional mutagenesis are indicated by a bold line below the region that was amplified. Plasmids pHW1, pHW2, and pHW3 contain inserts that span the intergenic regions indicated, while the insertional plasmid pHW4 contains a fragment internal to the ylxL gene.